The present invention relates to a dual polarized antenna element and, more particularly to a dual polarized antenna element comprising a planar radiating element arranged in a first plane, a metallic surface arranged in a second plane, which has at least two slots extending under the radiating element, and a supply conductor structure arranged in a third plane and coupled with the slots in the second plane which is above the third plane, in which one of the slots extends on a line of symmetry of the radiating element and at least one of the other slots runs perpendicularly to and mirror symmetric to the line of symmetry, whereby the slots are coupled with separate supply conductors.
Planar antennas, comprising a plurality of such dual polarized antenna elements, for example can be used in a base station and a subscriber's station of a point-to-multipoint radio communication system. This sort of planar antenna element has the advantage of a great flexibility in antenna characteristics during broadcasting, and may be manufactured economically in large numbers. In the point-to-multipoint radio communication system the radio link area about the base station is divided into several sectors, in which an individual planar antenna is present for each radio link sector. In order to obtain the greatest possible transmission capacity, the transmission channels in neighboring sections or also the transmission and reception channels within a sector carry different polarizations. When the use of separate transmission and reception antennas is avoided to save space, an antenna with dual polarized antenna elements can be used for both transmission and reception.
A dual polarized antenna element of the triggered type is described in Electronics Letters 31, No. 4, pp.245-246(Feb. 16, 1995). Besides a slot extending on a line of symmetry of the radiating element this antenna element has two other slots extending perpendicular to the opposing edges of the radiating element. The slot extending on the line of symmetry is coupled with a planar supply conductor for waves of a first polarization and a second supply conductor for waves of a second polarization is coupled with the other slots by means of a branching conductor. The dual polarized antenna element thus conceived has however a relatively high cross-polarization of between 32 and 35 dB.